For many years mattresses have been constructed with innerspring assemblies with multiple wire-form springs, covered by several layers of insulating, padding and upholstery material. For example, a fiber mat layer is secured directly over the tops of the springs, one or more intermediate foam layers above the fiber layer, a backing or ticking layer over the foam layer and connected to the innerspring assembly, and one or more upholstery layers which may also include bunting or other padding.
One performance deficiency associated with mattresses of this type of construction is a very noticeable and abrupt transition from the relatively soft and easily compressible upholstery and padding layers to the relatively stiff innerspring assembly. These is a result of the large difference in the nature of the combined materials, going from layers of fabric, foam and fibers, to wire-form steel. Even with additional layers of foam or fiber padding the harsh transition may not be avoided, owing to the relatively low density and easy compressibility of the layers. The use of low density, low ILD foams as padding layers gives a mattress a high degree of initial softness, but does not avoid the board-like firmness when the foam layers are quickly and fully compressed upon the innerspring assembly, causing the mattress to "bottom out" under high loadings.